This invention relates in general to the manufacture of polyimide foam products and, more specifically, to the improvement in density uniformity in such products.
Polyimide foam has been used for some time for thermal insulation, cushions, structural members and the like. Polyimides are particularly desirable where high temperature resistance is required. They also have the feature, highly desirable in aircraft, spacecraft, ships, and other enclosed spaces where people are present, of producing essentially no toxic gases at high temperatures or when exposed to direct flame.
A variety of polyimide foam materials have been developed such as those disclosed by Gagliani et al in U.S. Pat. Nos. 4,518 717 and Shulman et al in U.S. Pat. 4,467,597. Typical applications of polyimide foams to high temperature insulation structures are detailed in copending U.S. Pat. applications Ser. No. 07/312/490 filed 02/21/89 now U.S. Pat. No. 4,865,784 and Ser. No. 07/167,796 filed 03/17/89 now U.S. Pat. No. 4,874,648 by Crosswhite et al.
While excellent results have been obtained with polyimide foams, several problems have been encountered which affect the quality and size of the final product. During foaming of the product, particularly with microwave heating, excess gases are often generated in localized areas causing voids or vents. Also, during foaming, swirling occurs which prevents the foam having uniform local density throughout the bun, with significant variations in cell size. A crust or rind forms on the outside of the bun, which is unusable in the product and must be trimmed away. There is considerable waste in trimming away the rind and non-uniform regions of the foam buns.
Thus, there is a continuing need for improved methods of manufacturing polyimide foam products which avoid non-uniform density regions and eliminates wasting large portions of the foamed buns due to non-uniform regions and rinds.